This invention relates to a method and system for estimating the angular velocity of an electric power steering motor for an automobile.
An electric power steering apparatus for a motor vehicle typically uses an electric motor to assist an operator in applying the necessary torque required to steer the vehicle. When the vehicle is steered with a steering wheel operably connected to a set of road wheels, a sensor in the electric power steering apparatus detects the angular position and/or velocity of the motor. A signal is then sent from the sensor to an electric controller. The electric controller controls the magnitude and direction of the steering assist provided by the electric motor. The electric motor drives a reducing gear, typically a worm gear engaging a worm wheel, that lessens the torque required to turn the steering wheel. Electric steering systems often utilize a motor with a digital encoder for a position sensor. It is desireable to provide an electric power steering system that optimizes the estimation of the angular velocity of a motor at low and high velocities.
In an exemplary embodiment of the invention a method of optimizing the estimation of the angular velocity of an electric power steering motor for an automobile is disclosed. The method comprises estimating an average angular velocity of the motor; estimating an instantaneous angular velocity of the motor; weighting the estimated average angular velocity of the motor; weighting the estimated instantaneous angular velocity of the motor; and combining the weighted estimated average angular velocity of the motor and the weighted estimated instantaneous angular velocity of the motor.
The motor position signal can be used to estimate the angular velocity of the motor, by determining the change in angular position of the motor divided by the change in time. Additionally the motor velocity may be directly measured by a tachometer or other such device.
Given the discrete nature of the digital motor position signal, the resulting velocity signal can be computed using one of two fundamental techniques. The first involves measuring the motor angular change and dividing by a set period of time. The second involves measuring the amount of time between a known motor angle change. The first method yields good results when the motor is rotating at a relatively high velocity and the second method yields good results when the motor is rotating at a relatively low velocity.
If a motor velocity sensor is used, the optimization of the sensing system may utilize the motor velocity sensor at low motor velocity operation and a position based signal at high velocities
By utilizing different velocity estimation techniques for high motor velocity and low motor velocity, the estimation of the angular velocity of the electric motor may be optimized for bandwidth, resolution and accuracy over a prescribed speed range.